New research reveals unprecedented weakening of Asian summer monsoon

Precipitation from the Asian summer monsoon has declined over the last 80 years, an unprecedented decline over the past 448 years, according to a new study.

The new research used tree records to replenish the Asian summer monsoon until 1566. The study, published in the AGU journal Geophysical Research Letters, found that the monsoon had weakened since the 1940s, resulting in droughts and regional difficulties.

The new study shows that air pollutants of human origin are probably at the origin of the decline. According to the study's authors, the 80-year decline in the monsoon coincides with the current boom in industrial development and aerosol emissions in China and the northern hemisphere that began. around the end of the Second World War.

Previous studies have examined tree ring chronologies in this region, but the new study "goes beyond [previous dendrochronology studies] Steve Leavitt, a dendrochronologist at the University of Arizona in Tucson and co-author of the new study, said Steve Leavitt, co-author of the new study. "We were able to gather nearly 450 years of tree rings. data with a clear annual resolution from an area where growth of tree rings is very strongly correlated with rainfall. "

Nearly half of the world's population is affected by the Asian summer monsoon, which absorbs most of the rainfall on the continent in a few torrential months. Summer rainfall has declined in recent decades, affecting the availability of water, ecosystems and agriculture from India to Siberia.

The instrumental recordings and observation of monsoon strength and annual rainfall date back only about one hundred years. Long-term paleoclimatic records are needed to help determine whether this decline is due to anthropogenic factors such as aerosol pollution or natural variation in the monsoon cycle.

Trees record difficult times

The new study uses a set of 10 tree ring chronologies collected on the Loess Plateau in north-central China to track precipitation trends over the last 448 years. In the wetter years, trees tend to grow in dark circles and rainfall records can be gleaned by measuring the thickness and density of individual layers.

"One of the main benefits of using dark circles to study rainfall is the annual resolution and the exact dating," Leavitt said.

Tree rings captured periods of drought such as those of 1928 and 1929, which resulted in a widespread famine, killing more than 500,000 people in China alone. The results were also cross-checked with historical Chinese data for Desert Locust invasions, which tend to occur during drought years, as well as previously published chronologies for tree rings.

The new study found that the 80-year rainfall trend is unprecedented over the past 450 years, with thinner growth rings in the past 80 years than any other period. Before the 1940s, drought periods tended to be intermittent and shorter, with no decline in decades since 1566.

Several factors could affect the strength of the Asian summer monsoon, including solar variability, volcanic eruptions, and anthropogenic aerosols. The study authors used climate models to show that sulphate aerosols, air pollutants responsible for fog, are probably the dominant forcing agent controlling the decline of the Asian summer monsoon over the past 80 years. .

Liviu Giosan, a paleoclimatologist at the Woods Hole Oceanographic Institution in Massachusetts, said this study was an important data point in the ongoing quest to better understand the past and future of global monsoon systems that generate much of the world's rainfall. the new study.

"The monsoon is notoriously difficult to model and predict because of the high degree of regional variability," he said.

"To know more about the future, we need to better understand the past," said Giosan. "More of that kind of studies that show [monsoon activity] over entire regions will help us better understand the synoptic functioning of the Asian summer monsoon on the whole continent. "